1. Field of the Invention
This invention relates to electrochromic devices and, more particularly, to solid state electrochromic devices.
2. Art Background
Electrochromics have potentially significant advantages as display devices, and are presently being contemplated for use in large area displays. For example, the electrochemical reactions relied on in electrochromic devices usually require only small amounts of power while providing relatively large contrast between the bleached and colored state. The small power requirements involved in operating electrochromic devices limit complications in contemplated circuitry design for large scale arrays. Additionally, since electrochromics rely on electrically induced chemical changes, such devices usually exhibit memory. That is, once a chemical change is initiated by an appropriate electrical signal, the induced chemical state with a different coloration than the initial state is typically very stable under open circuit conditions. The color does not change until a suitable electrical impulse is supplied which reverses the chemical reaction. This memory property is often essential for many applications such as indicator displays.
The stability provided by an electrically induced chemical reaction, at least a priori, also offers the possibility of long term reliability. However, secondary effects often defeat this inherent long term stability. A common electrically induced reaction undergone by electrochromic materials having an oxidation state M.sup.n is represented by the equation EQU M.sup.n +e.sup.- +A.sup.+ .fwdarw.A.sup.+ M.sup.n-1
A device based on an electrochromic of this type requires an electrode to supply electrons to the electrochromic and a second electrode to supply a positive charge in the form of an ion, A.sup.+. One configuration of electrochromic device depending on this reaction mechanism utilizes solid electrodes immersed in a liquid electrolyte which acts as the electrochromic material. (See for example IEEE Trans. Electron Devices ED-22 (9), 749 (1975) disclosing the use of polytungsten anion as the electrolyte/electrochromic). The use of voltages above approximately 1.5 V are often necessary to yield suitable response times and in these cells can cause extensive electrolysis of the water with associated gas evolution. This side reaction generally causes serious packaging and stability problems in the finished device. Additionally, since the electrochromic is a liquid, the ions which undergo the color change easily migrate away from the electrode where coloration is induced. This migration during the interval between coloration and bleaching extends the bleaching response by the time necessary for return of these ions to the appropriate electrode.
Another secondary effect involves corrosion of the electrochromic material in devices which employ a separate solid electrochromic, and a liquid electrolyte. For example, devices with tungsten bronzes generally have a sulfuric acid electrolyte as a source of ions. The acidic electrolyte dissolves the tungsten bronze electrochromic material and ultimately causes failure of the cell.
The use of an all solid state device is being investigated in an attempt to eliminate or minimize the problems associated with devices having liquid components while retaining the advantageous properties potentially offered by electrochromics. Although solid state electrochromic devices have potentially significant advantages, many complications introduced by utilizing solids have not yet been overcome. For example, phosphotungstic acid (PWA) has been reported in display devices employing liquid electrolytes. See D. P. Hamblen, Research Disclosure, November 1977, pp. 58-59 No. 16347. However, investigations concerning the electrical properties of solid phosphotungstic acid (PWA)--in a context outside display technology--indicate that solid PWA spontaneously colors when contacted with metals--common sources of ions--(see O. Nakamura et al, Chubu Subsection Autumn Meeting at Nagoya University (1975)) and thus, cannot be used in an electrochromic configuration which relies on the reaction occurring only when a potential is introduced.
Other problems also can arise from the use of solids in a device. In most solid electrochromic devices an intermediary ionic conductor must be introduced between the source of ions and the electrochromic material which is a mixed electronic and ionic conductor. Configurations are utilized which include the successive elements of a first electrode assembly, an electrochromic material, an ionic conductor and a second electrode assembly. The ionic conductor provides conduction path only for ions and not for electrons. After the electrochromic material is colored by injection of electrons from the first electrode assembly, the reverse reaction, bleaching, could not be induced without the presence of the ionic conductor layer. If this layer is absent, during bleaching electrons would be withdrawn through the first electrode assembly but injected through the second electrode assembly. No net change in the colored state of the electrochromic would occur. The ionic conductor prevents injection of electrons from the second electrode assembly and ensures electrons are removed from the electrochromic material during bleaching. Additional layers substantially mitigate the fabrication advantages which might be obtainable in solid state devices.
A solid state device has been produced which does not have the undesirable intermediary layers of the other solid state electrochromic devices. The device utilizes between two metal electrodes a layer of SrTiO.sub.3 which is doped with electrochromic entities such as Mo and Ni to concentrations in the range of 10.sup.17 cm.sup.-3 to 10.sup.19 cm.sup.-3. (See Phys. Rev. B4, 3548 (1971)). These dopants undergo an electrically induced coloration. The cell must also be operated at temperatures above 200 degrees C. and at high voltages to obtain sufficient ionic conductivity through the doped SrTiO.sub.3. The necessity of high temperature and voltage operation makes practical applications of this device difficult.
The inherent limitations involved in devices utilizing a liquid medium have not been solved. Attempts to produce an all-solid-state device and maintain the desirable properties of these devices generally have been unsatisfactory.